This invention relates to an absorption chiller and, in particular to a method of controlling the capacity of an absorption chiller using two control variables to enable the chiller to operate close to the crystallization limit of the solution.
Heretofore, the temperature of the chilled water leaving the evaporator of an absorption machine was used as the sole variable to control the capacity of the machine. In the event the concentration of the solution leaving the lowest stage generator of the machine moved to one percent of the calculated crystallization concentration, the burner valve to the upper stage generator was prevented from opening any further. If the concentration continued to increase to within 0.6 percent of the crystallization limit, the heat input to the generator was reduced to about 67 percent of its previous setting and the burner was held at the 67 percent setting for a given period of time, usually about five minutes. This allowed the chilled water temperature to rise and the solution concentration leaving the lowest stage generator of the machine to be reduced sufficiently so that normal control over the burner could be once again resumed.
Although, this capacity control method works well under most operating conditions, there are times where the load demands on the chiller are such that the burner must be cycled at relatively short intervals to prevent crystallization of the solution. Accordingly, the chilled water leaving temperature changes repeatedly during this period. This repeated cycling of the burner may adversely effect performance and under certain conditions can waste energy.
It is therefore a primary object of this invention to improve absorption chillers.
It is a further object of this invention to more smoothly control the capacity of an absorption chiller as the solution concentration approaches the solution crystallization limit.
A still further object of this invention is to maintain the chilled water leaving temperature relatively constant during periods where the solution concentration of an absorption chiller approaches the solution crystallization limit.
Another object of the present invention is to use more than one variable to control the burner of an absorption chiller to permit the chiller to operate efficiently when the solution concentration is close to the crystallization limit.
These and other objects of the invention are attained by a capacity control procedure that includes measuring the chilled water temperature leaving the evaporator of an absorption chiller and controlling the burner of the chiller in response thereto until such time as the solution concentration being returned to the absorber approaches the solution crystallization limit. At this time control of the burner is changed over to respond to the concentration level of the solution to maintain the concentration at a steady desired setpoint below the crystallization limit. When the load condition changes to a point where the solution concentration is a given percentage away from the solution crystallization limit control of the burner is returned to the chilled water leaving temperature.